Polymer electrolyte and preparation method thereof

ABSTRACT

Provided are a polymer electrolyte membrane used in fuel cells, and a method for producing the same, the method including a step of filling a crosslinkable ion conductor in the pores of a porous nanoweb support; and a step of crosslinking the ion conductor filled in the pores of the porous nanoweb support. The method for producing a polymer electrolyte membrane uses a relatively smaller amount of an organic solvent, can ameliorate defects of the support caused by solvent evaporation, and can enhance the impregnability of the ion conductor to the support and the convenience of the process.

TECHNICAL FIELD

The present invention relates to an electrolyte used in fuel cells, andmore particularly, to a polymer electrolyte membrane for fuel cells.

BACKGROUND ART

Fuel cells are cells that directly convert the chemical energy generatedby oxidation of a fuel to electrical energy, and attention is being paidto the fuel cells as a next-generation energy source due to theirenvironment-friendly features of high energy conversion efficiency andreduced contaminant discharge.

A fuel cell generally has a structure in which an anode and a cathodeare formed, with an electrolyte membrane interposed therebetween, andsuch a structure is called a membrane-electrode assembly (MEA).

Fuel cells can be classified into alkaline electrolyte fuel cells,polymer electrolyte membrane fuel cells (PEMFC), and the like, and amongthem, polymer electrolyte membrane fuel cells are attracting moreattention as power source apparatuses for portable, automobile anddomestic applications, due to their advantages such as a low operationtemperature such as below 100° C., fast starting and fast responsecharacteristics, and excellent durability.

A representative example of such a polymer electrolyte membrane fuelcell is a proton exchange membrane fuel cell (PEMFC) that uses hydrogengas as the fuel.

To briefly describe the reaction occurring in a polymer electrolytemembrane fuel cell, first, when a fuel such as hydrogen gas is suppliedto an anode, an oxidation reaction of hydrogen occurs at the anode, andthereby hydrogen ions (H⁺) and electrons (e⁻) are produced. The hydrogenions (H⁺) thus produced are transferred to a cathode through a polymerelectrolyte membrane, and electrons (e⁻) thus produced are transferredto the cathode through an external circuit. Oxygen is supplied to thecathode, and oxygen binds with hydrogen ions (H⁺) and electrons (e⁻),and water is produced by a reduction reaction of oxygen.

Since the polymer electrolyte membrane is a channel through which thehydrogen ions (H⁺) produced at the cathode are transferred to thecathode, the polymer electrolyte membrane should essentially haveexcellent conductivity for hydrogen ions (H⁺). Furthermore, the polymerelectrolyte membrane should have excellent separation capability ofseparating hydrogen gas that is supplied to the anode and oxygen that issupplied to the cathode, and should also have excellent mechanicalstrength, dimensional stability, chemical resistance and the like, andcharacteristics such as a small ohmic loss at a high current density arerequired.

One of those polymer electrolyte membranes that are currently in use maybe an electrolyte membrane made of a perfluorosulfonic acid resin as afluororesin (hereinafter, referred to as “fluorine ion conductor”).However, a fluorine ion conductor has weak mechanical strength, and thushas a problem that when used for a long time, pinholes are generated,and thereby the energy conversion efficiency is decreased. In order toincrease the mechanical strength, there has been an attempt of using afluorine ion conductor having an increased membrane thickness; however,in this case, there is a problem that the ohmic loss is increased, andthe use of expensive materials increases, so that the economicefficiency is low.

In order to address such problems, there has been suggested a polymerelectrolyte membrane having enhanced mechanical strength by impregnatinga porous polytetrafluoroethylene resin (trade name: TEFLON)(hereinafter, referred to as “Teflon resin”), which is a fluororesin,with a liquid-state fluorine ion conductor. In this case, the hydrogenion conductivity may be somehow inferior as compared with polymerelectrolyte membranes composed of a fluorine ion conductor alone;however, the impregnated polymer electrolyte membrane is advantageous inthat the polymer electrolyte membrane has relatively superior mechanicalstrength, and can therefore have a reduced thickness, so that the ohmicloss is decreased.

However, since a Teflon resin has very poor adhesiveness, there arelimitations on the selection of the ion conductor, and in the case ofproducts produced by applying fluorine ion conductors, the products havea disadvantage that the fuel crossover phenomenon occurs conspicuouslyas compared with hydrocarbon-based ion conductors. Furthermore, becausenot only fluorine ion conductors but also porous Teflon resins arehighly expensive, there still is a demand for the development of a newinexpensive material for the mass production of fuel cells.

DISCLOSURE Technical Problem

An object of the present invention is to provide a method for producinga polymer electrolyte membrane, which uses a relatively smaller amountof an organic solvent, can improve defects of the support caused bysolvent evaporation, and can enhance the ability of the ion conductor toimpregnate the support, and the convenience of the process.

Another object of the present invention is to provide a polymerelectrolyte membrane having enhanced dimensional stability and tensilestrength.

Technical Solution

In order to achieve the objects described above, the method forproducing a polymer electrolyte membrane according to an aspect of thepresent invention includes a step of filling a crosslinkable ionconductor in the pores of a porous nanoweb support; and a step ofcrosslinking the ion conductor filled in the pores of the porous nanowebsupport. The crosslinking of the ion conductor may be carried out byapplying heat at a temperature of 150° C. to 200° C.

The crosslinkable ion conductor may be a low molecular weight ionconductor having a weight average molecular weight of 1,000 g/mol to50,000 g/mol.

The crosslinkable ion conductor may contain any one crosslinkablesubstituent selected from the group consisting of a monovalent aliphatichydrocarbon containing a carbon-carbon triple bond, a monovalentaliphatic hydrocarbon containing a carbon-carbon double bond, an epoxygroup, and combinations thereof, at any one position selected from thegroup consisting of the main chain, the chain ends, and combinationsthereof.

The crosslinkable ion conductor may be any one selected from the groupconsisting of a sulfonated polyimide (S-PI), a sulfonatedpolyarylethersulfone (S-PAES), a sulfonated polyether ether ketone(S-PEEK), a sulfonated polybenzimidazole (S-PBI), a sulfonatedpolysulfone (S-PSU), a sulfonated polystyrene (S-PS), a sulfonatedpolyphosphazene, and combinations thereof.

The crosslinkable ion conductor may be a compound represented by thefollowing formula (1):

wherein in the formula (1), SAr₁ represents a divalent sulfonatedaromatic hydrocarbon; Ar₁ and Ar₂ each independently represent adivalent aromatic hydrocarbon; X represents a divalent aromatichydrocarbon containing a crosslinkable substituent; Z₁ and Z₂ eachindependently represent a crosslinkable substituent or a monovalentaromatic hydrocarbon containing a crosslinkable substituent; a, c and deach independently represent a number from 0 to 0.999; b represents anumber from 0.001 to 1.000; and n represents an integer from 10 to 500.

SAr₁ may be any one selected from the group consisting of compoundsrepresented by the following formulas (2-1) to (2-4):

Ar₁ and Ar₂ each independently represent any one selected from the groupconsisting of compounds represented by the following formulas (3-1) and(3-2):

X may be any one selected from the group consisting of compoundsrepresented by the following formulas (4-1) and (4-2):

Z₁ and Z₂ each independently represent any one selected from the groupconsisting of compounds represented by the following formulas (5-1) to(5-4):

In the formulas (2) to (5), B₁ to B₃ each independently represent anyone selected from the group consisting of a halogen group, an alkylgroup having 1 to 5 carbon atoms, and a perfluoroalkyl group having 1 to5 carbon atoms; R′₁ and R′₂ each independently represent any oneselected from the group consisting of

M⁺ represents a counter cation having a monovalent positive charge; e₁represents an integer of 0 or 1; f₁ represents an integer from 1 to 3;e₂ represents an integer of 0 or 3; f₂ represents an integer from 1 to3; e₃ represents an integer of 0 or 4; f₃ represents an integer from 1to 4; g₁ to g₃ each independently represent an integer from 0 to 4; h₁to h₆ each independently represent an integer from 0 to 4; i representsan integer from 0 to 2; Y₁ and Y₂ each independently represent any oneselected from the group consisting of a single bond,

and R₁ to R₅ each independently represent any one selected from thegroup consisting of a hydrogen atom, a halogen group, an alkyl grouphaving 1 to 5 carbon atoms, and a perfluoroalkyl group having 1 to 5carbon atoms.

According to another aspect of the present invention, there is provideda polymer electrolyte membrane including a porous nanowet support, andan ion conductor that is filled inside the porous nanoweb support and iscrosslinked.

The crosslinked ion conductor may be any one selected from the groupconsisting of a crosslinked sulfonated polyimide, a crosslinkedsulfonated polyarylethersulfone, a crosslinked sulfonated polyetherether ketone, a crosslinked sulfonated polybenzimidazole, a crosslinkedsulfonated polysulfone, a crosslinked sulfonated polystyrene, acrosslinked sulfonated polyphosphazene, and combinations thereof.

The crosslinked ion conductor may be a crosslinked product of a compoundrepresented by the following formula (1):

wherein in the formula (1), SAr₁ represents a divalent sulfonatedaromatic hydrocarbon; Ar₁ and Ar₂ each independently represent adivalent aromatic hydrocarbon; X represents a divalent aromatichydrocarbon containing a crosslinkable substituent; Z₁ and Z₂ eachindependently represent a crosslinkable substituent a monovalentaromatic hydrocarbon containing a crosslinkable substituent; a, c and deach independently represent a number from 0 to 0.999; b represents anumber from 0.001 to 1.000; and n represents an integer from 10 to 500.

SAr₁ may be any one selected from the group consisting of compoundsrepresented by the following formulas (2-1) to (2-4):

Ar₁ and Ar₂ each independently represent any one selected from the groupconsisting of compounds represented by the following formulas (3-1) and(3-2):

X may be any one selected from the group consisting of compoundsrepresented by the following formulas (4-1) and (4-2):

Z₁ and Z₂ each independently represent any one selected from the groupconsisting of compounds represented by the following formulas (5-1) to(5-4):

In the formulas (2) to (5), B₁ to B₃ each independently represent anyone selected from the group consisting of a halogen group, an alkylgroup having 1 to 5 carbon atoms, and a perfluoroalkyl group having 1 to5 carbon atoms; R′₁ and R′₂ each independently represent any oneselected from the group consisting of

M⁺ represents a counter cation having a monovalent positive charge; e₁represents an integer of 0 or 1; f₁ represents an integer from 1 to 3;e₂ represents an integer of 0 or 3; f₂ represents an integer from 1 to3; e₃ represents an integer of 0 or 4; f₃ represents an integer from 1to 4; g₁ to g₃ each independently represent an integer from 0 to 4; h₁to h₆ each independently represent an integer from 0 to 4; i representsan integer from 0 to 2; Y₁ and Y₂ each independently represent any oneselected from the group consisting of a single bond,

and R₁ to R₅ each independently represent any one selected from thegroup consisting of a hydrogen atom, a halogen group, an alkyl grouphaving 1 to 5 carbon atoms, and a perfluoroalkyl group having 1 to 5carbon atoms.

The porous nanoweb support may contain any one selected from the groupconsisting of nylon, polyimide, polybenzoxazole, polyethyleneterephthalate, polyethylene, polypropylene, polytetrafluoroethylene,polyarylene ether sulfone, polyether ether ketone, copolymers thereof,and combinations thereof.

The porous nanoweb support may contain nanofibers having an averagediameter of 0.005 μm to 5 μm.

The porous nanoweb support may have a porosity of 50% to 98%, and mayhave pores having an average diameter of 0.05 μm to 30 μm.

Advantageous Effects

The method for producing a polymer electrolyte membrane according to thepresent invention uses a relatively smaller amount of an organicsolvent, can ameliorate the defects of the support caused by solventevaporation, and can enhance the impregnability of the ion conductor tothe support and the convenience of the process. Furthermore, the polymerelectrolyte membrane according to the present invention has enhanceddimensional stability and tensile strength.

DESCRIPTION OF DRAWINGS

FIG. 1 is a process flow diagram showing the method for producing apolymer electrolyte membrane according to an embodiment of the presentinvention.

BEST MODE

Hereinafter, the present invention will be described in more detail.

The definitions of the terms used in the present specification are asfollows.

Unless particularly stated otherwise herein, the alkyl group encompassesa primary alkyl group, a secondary alkyl group and a tertiary alkylgroup.

Unless particularly stated otherwise herein, the perfluoroalkyl groupmeans an alkyl group in which a portion of the hydrogen atoms or all thehydrogen atoms have been substituted by fluorine atoms.

Unless particularly stated otherwise herein, the halogen group means anyone selected from a fluorine atom, a chlorine atom, a bromine atom, andan iodine atom.

Unless particularly stated otherwise herein, all the compounds orsubstituents may be substituted or unsubstituted. Here, the term“substituted” means that a hydrogen atom in such a compound orsubstituent has been replaced by any one selected from the groupconsisting of a halogen atom, a hydroxyl group, a carboxyl group, acyano group, a nitro group, an amino group, a thio group, a methylthiogroup, an alkoxy group, a nitrile group, an aldehyde group, an epoxygroup, an ether group, an ester group, a carbonyl group, an acetalgroup, a ketone group, an alkyl group, a perfluoroalkyl group, acycloalkyl group, a heterocycloalkyl group, an allyl group, a benzylgroup, an aryl group, a heteroaryl group, derivatives thereof, andcombinations thereof.

Unless particularly stated otherwise herein, the aliphatic hydrocarbonis a hydrocarbon compound having 1 to 30 carbon atoms, which does notcontain a benzene ring, and the aliphatic hydrocarbon may have achain-like structure, a branched structure, or a non-aromatic cyclicstructure. Aliphatic hydrocarbons can be classified into alkanes inwhich the molecular chain contains single bonds only, alkenes in whichthe molecular chain contains doubles bonds, alkynes in which themolecular chain contains triple bonds, and the like. Examples of thealiphatic hydrocarbon include chain-like hydrocarbons, higher fattyacids, and esters thereof.

Unless particularly stated otherwise herein, the aromatic hydrocarbonmeans a monocyclic or polycyclic compound having 6 to 30 carbon atomsand containing one or more benzene ring, or a derivative thereof, andexamples thereof include phenyl compounds having a benzene ring;compounds having a benzene ring with alkyl branches, such as toluene andxylene; compounds having two or more benzene rings linked via a singlebond, such as biphenyl; compounds having a benzene ring fused with acycloalkyl group or a heterocycloalkyl group, such as fluorene, xantheneand anthraquinone; and compounds having two or more fused benzene rings,naphthalene and anthracene.

The polymer electrolyte membrane according to an embodiment of thepresent invention includes a porous nanoweb support, and an ionconductor that is filled in the inside of the porous nanoweb support andis crosslinked.

The porous nanoweb support is composed of aggregates of nanofibers thatare three-dimensionally connected in an irregular and non-continuousmanner, and therefore, the porous nanoweb support contains a largenumber of pores that are uniformly distributed. The porous nanowebsupport containing a large number of uniformly distributed pores hasexcellent gas or ion conductivity.

The pore diameter, which is the diameter of the pores formed in theporous nanoweb support, may be in the range of 0.05 μm to 30 μm. Ifpores are formed to have a pore diameter of less than 0.05 μm, the ionconductivity of the polymer electrolyte membrane may deteriorate, and ifthe pore diameter exceeds 30 μm, the mechanical strength of the polymerelectrolyte membrane may deteriorate.

Furthermore, the porosity that represents the extent of formation ofpores in the porous nanoweb support may be in the range of 50% to 98%.If the porosity of the porous nanoweb support is lower than 50%, the ionconductivity of the polymer electrolyte membrane may deteriorate, and ifthe porosity is higher than 98%, the mechanical strength andmorphological stability of the polymer electrolyte membrane maydeteriorate.

The porosity (%) can be calculated based on the ratio of the volume ofair relative to the total volume of the porous nanoweb support, as shownby the following mathematical formula (1):

Porosity(%)=(volume of air/total volume)×100  [Math FIG. 1]

At this time, the total volume of the porous nanoweb support iscalculated by producing a rectangular-shaped sample of the porousnanoweb support, and measuring the width, length and thickness of thesample. The volume of air in the porous nanoweb support may be obtainedby measuring the mass of the porous nanoweb support sample, and thensubtracting the polymer volume that has been calculated backward fromthe density, from the total volume of the porous nanoweb support sample.

The porous nanoweb support is composed of aggregates of nanofibers thatare three-dimensionally connected in an irregular and non-continuousmanner. The average diameter of the nanofiber may be in the range of0.005 μm to 5 μm. If the average diameter of the nanofiber is less than0.005 μm, the mechanical strength of the porous nanoweb support may bedecreased, and if the average diameter of the nanofiber is greater than5 μm, the porosity of the porous nanoweb support may not be easilycontrolled.

The nanofiber may be any one selected from the group consisting offibers made of nylon, polyimide, polybenzoxazole, polyethyleneterephthalate, polyethylene, polypropylene, polytetrafluoroethylene,polyarylene ether sulfone, polyether ether ketone, copolymers thereof,and combinations thereof, but the nanofiber is not intended to belimited to these.

The porous nanoweb support can be formed to have a thickness of 5 μm to20 μm. If the thickness of the porous nanoweb support is less than 5 μm,the mechanical strength and morphological stability of the polymerelectrolyte membrane may deteriorate, and if the thickness of the porousnanoweb support is greater than 20 μm, the ohmic loss of the polymerelectrolyte membrane may increase.

The ion conductor is a substance that implements the function of ionconduction, which is the principal function of a polymer electrolytemembrane, and as the ion conductor, a hydrocarbon-based polymer that hasexcellent ion conduction function and is advantageous in terms of pricecan be preferably used, but the ion conductor is not limited thereto.Particularly, for the facilitation of the process of filling the ionconductor inside the pores of the porous nanoweb support, a hydrocarbonsubstance which is soluble in organic solvents can be more preferablyused.

The ion conductor is an ion conductor obtained by filling a lowmolecular weight, crosslinkable ion conductor in the interior pores ofthe porous nanoweb support, and then crosslinking the ion conductor. Inthe case of filling a high molecular weight ion conductor in the porousnanoweb support, a method of preparing an ion conductor solution bydissolving the high molecular weight ion conductor in a large amount ofan organic solvent, and then soaking the porous nanoweb support in theion conductor solution, or applying the ion conductor solution on thesurface of the porous nanoweb support, or the like may be used. However,in this case, in the course of removing the organic solvent from thesolution, defects may be formed on the surface or in the interior of theporous nanoweb support, and such defects may cause an increase in thefilm resistance of the polymer electrolyte membrane and a decrease inthe film performance. Therefore, in order to prevent the defects, aprocess of removing the organic solvent in a vacuum at a hightemperature for a long time is needed.

On the other hand, since the ion conductor that is filled andcrosslinked inside the porous nanoweb support uses a small amount of anorganic solvent, and the organic solvent is removed during thecrosslinking process, the problem of defect generation is improved, andthe impregnability and the convenience of the process are enhanced.Furthermore, the ion conductor can be densely impregnated in theinterior of the porous nanoweb support. Furthermore, the dimensionalstability and tensile strength are improved as a result of thecrosslinking of the ion conductor.

The crosslinked ion conductor may be any one selected from the groupconsisting of a crosslinked sulfonated polyimide, a crosslinkedsulfonated polyarylethersulfone, a crosslinked sulfonated polyetherether ketone, a crosslinked sulfonated polybenzimidazole, a crosslinkedsulfonated polysulfone, a crosslinked sulfonated polystyrene, acrosslinked sulfonated polyphosphazene, and combinations thereof, butthe crosslinked ion conductor is not intended to be limited to these.

The crosslinked ion conductor may be a product obtained by filling acompound represented by the following formula (1) inside the porousnanoweb support, and then crosslinking the compound:

In the formula (1), SAr₁ represents a divalent sulfonated aromatichydrocarbon; Ar₁ and Ar₂ each independently represent a divalentaromatic hydrocarbon; X represents a divalent aromatic hydrocarboncontaining a crosslinkable substituent; and Z₁ and Z₂ each independentlyrepresent a crosslinkable substituent or a monovalent aromatichydrocarbon containing a crosslinkable substituent.

The symbols a, c and d each independently represent a number from 0 to0.999, and are each preferably a number from 0.001 to 0.900. brepresents a number from 0.001 to 1.000, and is preferably a number from0.098 to 0.997. n represents an integer from 10 to 500, and ispreferably an integer from 50 to 350.

In the formula (1), the symbol “/” is a symbol for distinguishing therespective units of the copolymer, and implies that the respective unitsmay be arranged regularly or irregularly within a repeating unit.

Specifically, SAr₁ may be any one selected from the group consisting ofcompounds represented by the following formulas (2-1) to (2-4):

In the formulas (2-1) to (2-4), M⁺ represents a counter cation having amonovalent positive charge, and may be any one selected from the groupconsisting of Na⁺, K⁺, an alkylammonium ion, and a proton.

The symbol e₁ represents an integer of 0 or 1; f₁ represents an integerfrom 1 to 3; e₂ represents an integer of 0 or 3; f₂ represents aninteger from 1 to 3; e₃ represents an integer of 0 or 4; and f₃represents an integer from 1 to 4.

Y₁ and Y₂ may be each independently any one selected from the groupconsisting of a single bond,

R₄ and R₅ may be each independently any one selected from the groupconsisting of a hydrogen atom, a halogen group, an alkyl group having 1to 5 carbon atoms, and a perfluoroalkyl group having 1 to 5 carbonatoms.

Ar₁ and Ar₂ may be each independently any one selected from the groupconsisting of compounds represented by the following formulas (3-1) and(3-2):

In the formulas (3-1) and (3-2), B₁ and B₂ may be each independently anyone selected from the group consisting of a halogen group, an alkylgroup having 1 to 5 carbon atoms, and a perfluoroalkyl group having 1 to5 carbon atoms, and are each preferably a fluoro group.

g₁ to g₃ each independently represent an integer from 0 to 4.

Y₁ has the same meaning as defined in the description of the compoundsrepresented by the formulas (2-1) to (2-4), and therefore, furtherspecific descriptions thereon will not be repeated here.

X may be any one selected from the group consisting of compoundsrepresented by the following formulas (4-1) and (4-2):

In the formulas (4-1) and (4-2), R′₁ and R′₂ may be each independentlyany one selected from the group consisting of

and R₁ to R₃ may be each independently any one selected from the groupconsisting of a hydrogen atom, a halogen group, an alkyl group having 1to 5 carbon atoms, and a perfluoroalkyl group having 1 to 5 carbonatoms; i represents an integer from 0 to 2.

Y₁ and Y₂ have the same meanings as defined in the description of thecompounds represented by the formulas (2-1) to (2-4), and therefore,further specific descriptions thereon will not be repeated here.

Z₁ and Z₂ each independently represent any one selected from the groupconsisting of compounds represented by the following formulas (5-1) to(5-4):

In the formula (5), B₁ to B₃ each independently represent any oneselected from the group consisting of a halogen group, an alkyl grouphaving 1 to 5 carbon atoms, and a perfluoroalkyl group having 1 to 5carbon atoms; h₁ to h₆ each independently represent an integer from 0 to4; R′ has the same meaning as defined in the description of thecompounds represented by the formulas (4-1) and (4-2), and therefore,further specific descriptions thereon will not be repeated here.

The method for producing a polymer electrolyte membrane according toanother embodiment of the present invention includes a step of filling acrosslinkable ion conductor in the pores of a porous nanoweb support,and crosslinking the ion conductor filled in the pores of the porousnanoweb support.

FIG. 1 is a process flow diagram showing the method for producing apolymer electrolyte membrane according to an embodiment of the presentinvention. Hereinafter, the method for producing a polymer electrolytemembrane will be described with reference to FIG. 1.

The method for producing a polymer electrolyte membrane includes a stepof producing a porous nanoweb support (S1); a step of dissolving acrosslinkable ion conductor in an organic solvent, and thereby preparingan ion conductor solution (S2); a step of filling the ion conductorsolution in the pores of the porous nanoweb support (S3); and a step ofcrosslinking the ion conductor (S4).

The step (S1) of producing a porous nanoweb support includes processesof dissolving a precursor in a spinning solvent to prepare a spinningsolution, spinning the spinning solution thus prepared to produce aporous nanoweb composed of nanofibers having an average diameter of0.005 μm to 5 μm, and then treating the nanoweb thus produced by apost-treatment.

It is preferable to produce the porous nanoweb support through anelectrospinning process in order to obtain a thin film having a highporosity and fine pores, but the process is not limited thereto.

The porous nanoweb support can be produced by spinning any one selectedfrom the group consisting of nylon, polyimide, polybenzoxazole,polyethylene terephthalate, polyethylene, polypropylene,polytetrafluoroethylene, polyarylene ether sulfone, polyether etherketone, copolymers thereof, and combinations thereof.

On the other hand, a porous nanoweb material which is insoluble inorganic solvents cannot be directly produced through an electrospinningprocess. That is, a polyimide or polybenzoxazole that can form a porousnanoweb does not easily dissolve in organic solvents such asN-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide(DMA) and dimethyl sulfoxide (DMSO), and accordingly, it is difficult toprepare a spinning solution from the materials.

Therefore, a porous nanoweb support which is insoluble in organicsolvents can be produced by first producing a precursor nanoweb using aprecursor that is highly soluble in an organic solvent, and thentreating the precursor nanoweb thus produced by a post-treatment so asto prevent the precursor nanoweb from dissolving in the organic solvent.

Examples of the post-treatment method for producing a non-soluble porousnanoweb from the precursor nanoweb support, include a heat treatmentmethod and a chemical treatment method. Particularly, the heat treatmentmethod can be carried out using a hot press set at a high temperatureand a high pressure.

The production of a porous nanoweb support using a polyimide may bespecifically described as follows.

A polyimide porous nanoweb support can be produced by electrospinning apolyamic acid precursor to form a nanoweb precursor, and then imidizingthe nanoweb precursor using a hot press.

More specifically, a polyimide porous nanoweb support can be produced bydissolving a polyamic acid in tetrahydrofuran (THF) solvent to prepare aprecursor solution; discharging the precursor solution through aspinning nozzle while a high voltage of 1 to 1,000 kV is applied at atemperature of 20° C. to 100° C., to form a polyamic acid nanoweb on acollector; and then heat treating the polyamic acid nanoweb with a hotpress set to a temperature of 80° C. to 400° C.

The porous nanoweb support can improve the heat resistance, chemicalresistance and mechanical properties of the polymer electrolytemembrane, as compared with polymer electrolytes produced only from ionconductors.

The crosslinkable ion conductor is filled in the pores of the porousnanoweb support (S3). Since the description on the crosslinkable ionconductor is the same as the description given for the polymerelectrolyte membrane according to an embodiment of the presentinvention, and therefore, further specific descriptions thereon will notbe repeated here.

However, the weight average molecular weight of the crosslinkable ionconductor may be 1,000 g/mol to 50,000 g/mol, and is preferably 5,000g/mol to 20,000 g/mol. When the weight average molecular weight of theion conductor is a low molecular weight in the range described above,the penetration of the ion conductor into the pores of the porousnanoweb support is facilitated, and the impregnability can be furtherimproved. In regard to the method of filling, an ion conductor solutioncan be prepared by dissolving the crosslinkable ion conductor in asolvent (S2), and then the ion conductor solution can be used to fill inthe pores (S3). In the case of utilizing the immersion process describedabove, it is preferable to carry out the immersion process two to fivetimes for 5 to 30 minutes at normal temperature.

For the process of filling the crosslinkable ion conductor in the poresof the porous nanoweb support, an immersion process or an impregnationprocess can be used, but the process is not limited thereto, and variousmethods that are known in the pertinent art, such as a laminationmethod, a spraying method, a screen printing method, and a doctor blademethod can be utilized.

Subsequently, the ion conductor filled in the pores of the porousnanoweb support is crosslinked (S4). The crosslinking of the ionconductor can be achieved by applying heat at a temperature of 150° C.to 200° C., and preferably 150° C. to 180° C. If the crosslinkingtemperature is higher than 200° C., sulfonic acid may be decomposed, andif the temperature is lower than 150° C., the crosslinking reaction maynot occur.

MODE FOR INVENTION

Hereinafter, the present invention will be described more specificallyby way of Examples and Comparative Examples. However, the followingExamples are only for illustrative purposes, and are not intended tolimit the scope of rights of the present invention by any means.

Preparation Example Preparation of Polymer Electrolyte Membrane Example1

A polyamic acid/THF spinning solution at a concentration of 12 wt % waselectrospun while a voltage of 30 kV was applied, and thereby a polyamicacid nanoweb precursor was formed. Subsequently, the nanoweb precursorwas heat treated in an oven at 350° C. for 5 hours, and thus a polyimideporous nanoweb support having an average thickness of 15 μm wasproduced. At this time, the electrospinning process was carried outusing a spray jet nozzle at 25° C. while a voltage of 30 kV was applied.The polyimide porous nanoweb support thus produced contained nanofibershaving an average diameter of 1 μm and pores having an average diameterof 2 μm, and had a porosity of 90%.

An ion conductor represented by the following formula (6) was dissolvedin N-methyl-2-pyrrolidone (NMP), and thus a 10 wt % ion conductorsolution was prepared.

wherein in the formula (6), there are 10 to 500 repeating units.

The porous nanoweb thus produced was immersed in the ion conductorsolution. Specifically, the immersion process was carried out threetimes for 20 minutes at normal temperature, and at this time, a reducedpressure atmosphere was applied for about one hour to eliminate fine gasbubbles. Thereafter, the ion conductor was crosslinked by heating theion conductor at 175° C., while NMP was removed. Thus, a polymerelectrolyte membrane was produced.

Example 2

A polymer electrolyte membrane was produced in the same manner as inExample 1, except that an ion conductor represented by the followingformula (7) was used as the ion conductor.

wherein in the formula (7), 0.3=x=1.0; and y=1−x.

Reference Example

A polyamic acid/THF spinning solution at a concentration of 12 wt % waselectrospun while a voltage of 30 kV was applied, and thereby a polyamicacid nanoweb precursor was formed. Subsequently, the nanoweb precursorwas heat treated in an oven at 350° C. for 5 hours, and thus a polyimideporous nanoweb support having an average thickness of 15 μm wasproduced. At this time, the electrospinning process was carried outusing a spray jet nozzle at 25° C. while a voltage of 30 kV was applied.Sulfonated polyether ether ketone (S-PEEK) was dissolved inN-methyl-2-pyrrolidone (NMP), and thus a 10 wt % ion conductor solutionwas prepared.

The porous nanoweb support thus produced was immersed in the ionconductor solution. Specifically, the immersion process was carried outthree times for 20 minutes at normal temperature, and at this time, areduced pressure atmosphere was applied for about one hour to eliminatefine gas bubbles. Thereafter, the immersed porous nanoweb support wasdried in a hot air oven maintained at 80° C. for 3 hours to remove NMP.Thus, a polymer electrolyte membrane was produced.

Experiment Example 1 Analysis of Properties of Produced Porous NanowebSupport

The properties of the porous nanoweb supports produced in the Examplesand Reference Example as described above were measured according to ASTM638. The results are presented in the following Table 1. At this time,specific measurement conditions are as follows.

-   -   Tensile speed: 25 cm/min    -   Grip distance: 6.35 cm    -   Temperature and humidity: 25° C.×50%

TABLE 1 Type Strength (MPa) Elongation (%) Elastic modulus (MPa) PIsupport 40 5 2500

Experiment Example 2 Analysis of Properties of Produced PolymerElectrolyte Membrane

The properties of the polymer electrolyte membranes produced in theExamples and Reference Example as described above were measured. Theresults are presented in the following Table 2.

1) Measurement of Water Uptake

Each of the polymer electrolyte membranes thus produced was immersed inultrapure water for 24 hours, and then was removed therefrom to measurethe weight of the membrane in a wet state (W_(wet)). An identicalpolymer electrolyte membrane was dried in a vacuum state at 100° C. for24 hours, and then the weight of the membrane in a dry state (W_(dry))was measured. Thus, the water uptake of the membrane was calculatedaccording to the following formula:

Water uptake(%)=((W _(wet) −W _(dry))/W _(dry))×100

2) Measurement of Swelling Ratio

Each of the polymer electrolyte membranes thus produced was immersed inultrapure water for 24 hours, and then was removed therefrom to measurethe area (l_(wet)) or thickness (t_(wet)) in a wet state. An identicalpolymer electrolyte membrane was dried in a vacuum at 100° C. for 24hours, and then the area (l_(dry)) or thickness (t_(dry)) in a dry statewere measured. Thus, the swelling ratio of the membrane was calculatedbased on the area and the thickness according to following formulas,respectively:

Swelling ratio(Δl,%)=((l _(wet) −l _(dry))/l _(dry))×100

Swelling ratio(Δt,%)=((t _(wet) −t _(dry))/t _(dry))×100

TABLE 2 Water Tensile Swelling ratio IEC uptake strength (%) Type(meq/g) (%) (MPa) Δt (%) Δl (%) Example 1 2.00 5 30 2 3 Example 2 2.00 735 3 3 Reference Example 2.00 17 20 15 7

According to the above Table 2, in the polymer electrolyte membraneproduced in the Reference Example, the dimensional stability in thethickness direction was not improved, due to the swelling of the surfacelayer. However, it can be seen that in the polymer electrolyte membranesproduced in Examples 1 and 2, since the surface layers were alsocrosslinked, swelling did not occur in the surface layers, and thedimensional stability was improved.

3) Fenton Test

Each of the polymer electrolyte membrane thus produced was immersed in aFenton's solution, which was prepared by 2 ppm of iron sulfateheptahydrate to a 3 wt % hydrogen peroxide solution, and then theoccurrence of deterioration of the membrane caused by radicals wasobserved at 80° C. for a determined time period. Thereafter, the weightsof the membrane before and after the immersion were measured. Theresults are presented in the following Table 3.

TABLE 3 Retention time Weight change Example 1 10 hours or longer Noweight change Example 2 10 hours or longer No weight change ReferenceExample 10 hours Not measurable

According to the results of the above Table 3, in the polymerelectrolyte membrane produced in the Reference Example, it wasimpossible to measure the weight because the membrane was degraded after10 hours of immersion in the Fenton's reagent. However, in the polymerelectrolyte membranes produced in Example 1 and Example 2, no weightchange was observed even after the membranes had been immersed for 10hours or more in the Fenton's reagent. From these results, it can beseen that the polymer electrolyte membranes according to Example 1 andExample 2 had excellent resistance to radicals, and thus exhibitedimproved dimensional stability.

1. A method for producing a polymer electrolyte membrane, the methodcomprising: filling a crosslinkable ion conductor in the pores of aporous nanoweb support; and crosslinking the ion conductor filled in thepores of the porous nanoweb support.
 2. The method for producing apolymer electrolyte membrane according to claim 1, wherein thecrosslinking of the ion conductor is carried out by applying heat at atemperature of 150° C. to 200° C.
 3. The method for producing a polymerelectrolyte membrane according to claim 1, wherein the crosslinkable ionconductor is a low molecular weight ion conductor having a weightaverage molecular weight of 1,000 g/mol to 50,000 g/mol.
 4. The methodfor producing a polymer electrolyte membrane according to claim 1,wherein the crosslinkable ion conductor contains any one crosslinkablesubstituent selected from the group consisting of a monovalent aliphatichydrocarbon containing a carbon-carbon triple bond, a monovalentaliphatic hydrocarbon containing a carbon-carbon double bond, an epoxygroup, and combinations thereof, at any position selected from the groupconsisting of the main chain, the chain ends, and combinations thereof.5. The method for producing a polymer electrolyte membrane according toclaim 1, wherein the crosslinkable ion conductor is any one selectedfrom the group consisting of a sulfonated polyimide (S-PI), a sulfonatedpolyarylethersulfone (S-PAES), a sulfonated polyether ether ketone,(S-PEEK), a sulfonated polybenzimidazole (S-PBI), a sulfonatedpolysulfone (S-PSU), a sulfonated polystyrene (S-PS), a sulfonatedpolyphosphazene, and combinations thereof.
 6. The method for producing apolymer electrolyte membrane according to claim 1, wherein thecrosslinkable ion conductor is a compound represented by the followingformula (1):

wherein in the formula (1), SAr₁ represents a divalent sulfonatedaromatic hydrocarbon; Ar₁ and Ar₂ each independently represent adivalent aromatic hydrocarbon; X represents a divalent aromatichydrocarbon containing a crosslinkable substituent; Z₁ and Z₂ eachindependently represent a crosslinkable substituent or a monovalentaromatic hydrocarbon containing a crosslinkable substituent; a, c and deach independently represent a number from 0 to 0.999; b represents anumber from 0.001 to 1.000; and n represents an integer from 10 to 500.7. The method for producing a polymer electrolyte membrane according toclaim 6, wherein SAr₁ represents any one selected from the groupconsisting of compounds represented by the following formulas (2-1) to(2-4):

Ar₁ and Ar₂ each independently represent any one selected from the groupconsisting of compounds represented by the following formulas (3-1) and(3-2):

X represents any one selected from the group consisting of compoundsrepresented by the following formulas (4-1) and (4-2):

Z₁ and Z₂ each independently represent any one selected from the groupconsisting of compounds represented by the following formulas (5-1) to(5-4):

wherein in the formulas (2) to (5), B₁ to B₃ each independentlyrepresent any one selected from the group consisting of a halogen group,an alkyl group having 1 to 5 carbon atoms, and a perfluoroalkyl grouphaving 1 to 5 carbon atoms; R′₁ and R′₂ each independently represent anyone selected from the group consisting of

M⁺ represents a counter cation having a monovalent positive charge; e₁represents an integer of 0 or 1; f₁ represents an integer from 1 to 3;e₂ represents an integer of 0 or 3; f₂ represents an integer from 1 to3; e₃ represents an integer of 0 or 4; f₃ represents an integer from 1to 4; g₁ to g₃ each independently represent an integer from 0 to 4; h₁to h₆ each independently represent an integer from 0 to 4; i representsan integer from 0 to 2; Y₁ and Y₂ each independently represent any oneselected from the group consisting of a single bond,

and R₁ to R₅ each independently represent any one selected from thegroup consisting of a hydrogen atom, a halogen group, an alkyl grouphaving 1 to 5 carbon atoms, and a perfluoroalkyl group having 1 to 5carbon atoms.
 8. A polymer electrolyte membrane comprising: a porousnanoweb support; and an ion conductor that is filled inside the porousnanoweb support and is crosslinked.
 9. The polymer electrolyte membraneaccording to claim 8, wherein the crosslinked ion conductor is any oneselected from the group consisting of a crosslinked sulfonatedpolyimide, a crosslinked sulfonated polyarylethersulfone, a crosslinkedsulfonated polyether ether sulfone, a crosslinked sulfonatedpolybenzimidazole, a crosslinked sulfonated polysulfone, a crosslinkedsulfonated polystyrene, a crosslinked sulfonated polyphosphazene, andcombinations thereof.
 10. The polymer electrolyte membrane according toclaim 8, wherein the crosslinked ion conductor is a crosslinked productof a compound represented by the following formula (1):

wherein in the formula (1), SAr₁ represents a divalent sulfonatedaromatic hydrocarbon; Ar₁ and Ar₂ each independently represent adivalent aromatic hydrocarbon; X represents a divalent aromatichydrocarbon containing a crosslinkable substituent; Z₁ and Z₂ eachindependently represent a crosslinkable substituent or a monovalentaromatic hydrocarbon containing a crosslinkable substituent; a, c and deach independently represent a number from 0 to 0.999; b represents anumber from 0.001 to 1.000; and n represents an integer from 10 to 500.11. The polymer electrolyte membrane according to claim 10, wherein SAr₁represents any one selected from the group consisting of compoundsrepresented by the following formulas (2-1) to (2-4):

Ar₁ and Ar₂ each independently represent any one selected from the groupconsisting of compounds represented by the following formulas (3-1) and(3-2):

X represents any one selected from the group consisting of compoundsrepresented by the following formulas (4-1) and (4-2):

Z₁ and Z₂ each independently represent any one selected from the groupconsisting of compounds represented by the following formulas (5-1) to(5-4):

wherein in the formulas (2) to (5), B₁ to B₃ each independentlyrepresent any one selected from the group consisting of a halogen group,an alkyl group having 1 to 5 carbon atoms, and a perfluoroalkyl grouphaving 1 to 5 carbon atoms; R′₁ and R′₂ each independently represent anyone selected from the group consisting of

M⁺ represents a counter cation having a monovalent positive charge; e₁represents an integer of 0 or 1; f₁ represents an integer from 1 to 3;e₂ represents an integer of 0 or 3; f₂ represents an integer from 1 to3; e₃ represents an integer of 0 or 4; f₃ represents an integer from 1to 4; g₁ to g₃ each independently represent an integer from 0 to 4; h₁to h₆ each independently represent an integer from 0 to 4; i representsan integer from 0 to 2; Y₁ and Y₂ each independently represent any oneselected from the group consisting of a single bond,

and R₁ to R₅ each independently represent any one selected from thegroup consisting of a hydrogen atom, a halogen group, an alkyl grouphaving 1 to 5 carbon atoms, and a perfluoroalkyl group having 1 to 5carbon atoms.
 12. The polymer electrolyte membrane according to claim 8,wherein the porous nanoweb support contains any one selected from thegroup consisting of nylon, polyimide, polybenzoxazole, polyethyleneterephthalate, polyethylene, polypropylene, polytetrafluoroethylene,polyarylene ether sulfone, polyether ether ketone, copolymers thereof,and combinations thereof.
 13. The polymer electrolyte membrane accordingto claim 8, wherein the porous nanoweb support contains nanofibershaving an average diameter of 0.005 μm to 5 μm.
 14. The polymerelectrolyte membrane according to claim 8, wherein the porous nanowebsupport has a porosity of 50% to 98%, and has pores having an averagediameter of 0.05 μm to 30 μm.